1. Field of the Invention
The present invention relates to a solid electrolytic capacitor, and more particularly to a cathode lead-out structure of the solid electrolytic capacitor and a method for fabricating the same.
2. Description of the Related Art
A conventional solid electrolytic capacitor of the kind to which the present invention relates is shown in FIG. 1A. In fabricating such capacitor, particles of valve-action metal such as tantalum and aluminum are pressure-molded into an anode body 1, in which an anode body lead 2 of valve-action metal is set and which is sintered in vacuum. A dielectric oxide coat layer 4 is formed on an outer peripheral surface of the anode body 1 by way of an oxidation process of the anode body. Then, a semiconductor oxide layer 5 of such as manganese dioxide is formed as an opposing electrode of the capacitor on an outer peripheral surface of the dielectric oxide coat layer 4, a graphite layer 7a for reducing contact resistance is formed on the semiconductor oxide layer 5, and a silver paste layer 8a is formed on the graphite layer 7a to complete the fabrication of the capacitor. A combination of the respective cathode layers, i.e., the semiconductor oxide layer 5, graphite layer 7a and silver paste layer 8a forms a combined cathode electrode of the capacitor.
The capacitor element fabricated as above suffers from such problems that, since strength of mechanical contact between the respective cathode layers is weak, the electrical conductivity deteriorates due to the development of cracks and separations caused by such conditions as high temperature, high humidity and high pressure, and accordingly the dielectric loss and the impedance are increased. As for suppressing the deterioration of the electric conductivity, Japanese Patent Application Kokoku Publication No. Hei 2-11009 discloses a fabrication method in which, as shown in FIG. 1B, the element formed up to the semiconductor oxide layer 5 using the same material and the same fabrication steps as those used in FIG. 1A is dipped in graphite paste into which particles of palladium are mixed, followed by a drying process thereby forming a palladium-containing graphite layer 7b. Then, non-electrolytic plating is provided on the palladium-containing graphite layer 7b thereby forming a non-electrolytic nickel plated layer 8b.
As already pointed out, the capacitor element fabricated as above suffers from problems such that, when used under high temperature, high humidity and high pressure conditions, there develop cracks and separations between the respective cathode layers, especially in the interface between the semiconductor oxide layer and the graphite layer, resulting in the deterioration of electrical conductivity and in the increase in the dielectric loss and the impedance.
Also, there is a further problem that the migration of metal or graphite particles into cracked or separated portions results in an increase in leakage currents.